Present television cameras are limited in useful light intensity differentials whereby a sensitive television camera, suitable for low light level input, will be overloaded or damaged by high light level inputs, requiring light reducing filters to be interposed. Conversely, high light level cameras will be insensitive to low light level inputs. The present invention provides a means for extending the useful light input range of a television camera.
Conventional photographic cameras and most television cameras are provided with lenses having variable aperture irises to vary the amount of light permitted to fall upon the photosensitive elements. Since irises are mechanical, they are limited in their range of light attenuation, with an iris range up to f/22 being common. Accordingly, other means of additional light attenuation is required. One method of reducing the amount of light passing through an iris at its maximum closure is by the use of a spot filter, that is, an optical element having a spot of greater filtering power in optical alignment with the iris. The effect of the relatively small filtering area is that, when the iris is at its maximum closure, all light passes through the spot filter thereby greatly attenuating the light which passes through to the photosensitive elements. An iris range of up to f/400 or f/500 is possible. Conversely, when the iris is fully opened, the effect of the relatively small filtering area, when placed close to the iris on the optical axis, is minimal, generally about 1/3 f stop. As the iris gradually closes, the effect of the spot becomes more and more pronounced and attenuation is greatly increased.
Many television cameras are provided with automatic iris drives to mechanically adjust the iris for desired light attenuation. It is accordingly important that, at any point throughout the iris range, the density gradient of the spot filter not become so steep as to cause any of the automatic lens control or automatic iris drives to be activated improperly. It will be seen that the density gradient of the spot filter must be carefully controlled.
The method of manufacture of the attenuation spot filter of the present invention provides the required careful control of the density gradient of the spot.